It has been known for some time that cationic water-soluble polymers can flocculate and remove suspended finely divided solids from water. This can be seen for example, in Weidner et al U.S. Pat. No. 2,995,512, and Wiley U.S. Pat. No. 3,055,827. Such cationic polymers made from the monomer dimethyl diallyl ammonium chloride are shown in Schuller et al U.S. Pat. No. 2,923,701, and in Butler's Canadian Pat. No. 945,432. See, also, Hoover et al U.S. Pat. No. 3,412,019 and Schaper U.S. Pat. No. 3,514,398.
Virtually all water intended to be used for drinking, as in a municipal water supply, is subjected to a clarification process to remove substances which impart turbidity and objectionable color to the water. In addition, most natural waters used for industrial purposes are subjected to a clarification process. These substances are generally nonsettling clay, bacteria, and colloidal silt-like materials. The nonsettling particles usually have a particle diameter of less than 10 microns, most often between 1.0 and 0.01 microns. Because of the small particle size, these materials cannot be removed by simple settling processes. The colloidal particles of the smaller size present the major problem because they are present to some extent at all times, and during certain periods are present in very high concentrations, as for example, during periods of climatic disturbances such as heavy rain and snow storms, high winds, and the like.
For all practical purposes, particles having a diameter of less than 10 microns cannot be removed by untreated settling alone. The smaller particles settle so slowly on their own that the retention time is excessive and makes the treatment economically unattractive. Therefore, coagulation has been used to remove these small diameter particles from natural waters. In coagulation, the fine particles are joined together to form larger and heavier particles (flocs) which are able to settle rapidly. In this way, water is obtained which is of sufficient clarity and purity to be acceptable for most industrial purposes and which only needs to be filtered and chlorinated to be of a potable quality. The flocculation process makes it possible to remove the fine particles which would not be removed by settling and which would pass through the filter medium (which must be somewhat coarse to permit acceptable flow rates). With waters of low turbidity, that is, having a Jackson Turbidity Unit value from 1.0 to 10.0, flocculation may be followed by filtration. However, for waters having higher turbidities, a settling step is necessary before filtration.
The most widely used coagulants in the past were aluminum or iron. Aluminum sulfate (filter alum) was the most widely used coagulant. Ferrous sulfate (copperas) was also used to a great extent. Ferric sulfate (ferrifloc of ferrisul), ferric chloride, and sodium aluminate have also been employed to some extent as coagulants. The chemical reactions which occur during coagulation with these inorganic coagulants are somewhat complex and involve not only the direct union of the coagulant ions with the impurities in the water, but they also involve the formation of hydrous oxides. The hydrous oxides of the metals are the flocs which ultimately precipitate and remove the objectionable contaminants from the water.
In addition, certain highly colloidal clays of the swelling bentonite type have been used for clarification. These clays are produced generally in Wyoming and South Dakota and are capable of forming thick gels many times the volume of the original bentonite when added to water. They are sometimes called "sodium bentonites" and their use for water purification purposes is fully described in U.S. Pat. No. 2,345,827 and 2,362,022.
Because of their availability and ease of use, the above-mentioned inorganic coagulants have been widely used. However, these inorganic coagulants do not form a very stable floc and, in addition, the flocs are not as large as is desirable for rapid settling. Therefore, it has become common to use a polyelectrolyte in conjunction with the inorganic coagulant or as a partial or total replacement for the inorganic coagulant. For example, Hronas, in U.S. Pat. No. 3,066,095, discloses the use of the inorganic coagulants along with a bentonitic clay and a polyacrylamide. See, also, Hedrick et al U.S. Pat. Nos. 3,516,932 and 3,637,491. In addition, see Wiley, U.S. Pat. No. 3,055,827, which discloses the use of a vinyl benzyl quaternary ammonium polymer. Other patents of interest in this area are Nagan, U.S. Pat. No. 3,131,144; Demeter, U.S. Pat. No. 3,350,302; Clark, U.S. Pat. No. 3,388,060; and Dajani, U.S. Pat. Nos. 3,408,292 and 3,409,547.
For certain purposes it is also known to combine the use of a cationic polymer with the use of an anionic polymer but without premixing them. See Priesing et al U.S. Pat. No. 3,259,570.
Sak, in U.S. Pat. No. 3,397,139, suggests the addition of both a cationic polymer and an anionic polymer at different points in a sludge treatment method.
To our knowledge, however, it has not been suggested to use a combination of cationic polymers such as homopolymers of polyethylene imine and/or dimethyl diallyl ammonium chloride and/or methacryloxyethyl trimethyl ammonium methosulfate combined with nonionic polyacrylamide and/or polyethylene oxide.
In particular, no one to our knowledge has discovered the increase in efficiency which may be imparted to the use of such cationics by the co-mixture therewith of from about 5 weight percent to about 20 weight percent of a nonionic polymer.